1. Field of the Invention
The present invention relates generally to the removal of vapors and particulates from a flowing gas stream, and more particularly to the removal of metal halides from waste streams, such as those resulting from the treatment of glass with a volatile metal halide such as anhydrous stannic chloride, titanium tetrachloride, or silicon tetrachloride.
To prevent damage during high speed filling operations, glass containers such as jars, bottles, and the like, are coated with various films which strengthen the glass during filling, subsequent handling, and ultimate use by the consumer. A particularly effective coating can be formed by spraying metal halides such as anhydrous stannic chloride (SnCl.sub.4) or anhydrous titanium tetrachloride (TiCl.sub.4) onto the glassware at a high temperature, which treatment results in the formation of a layer of the corresponding metal oxide on the surface of the glass. Such coatings have been found to substantially enhance the strength of the treated glass.
The anhydrous metal halides are typically sprayed onto the bottles which are at a very high temperature, typically in the range from 800.degree. F. to 1100.degree. F. On contact, the metal halides are converted to the corresponding metal oxide, resulting in the formation of HCl. A large proportion of the metal halide, spray, however, does not adhere to the bottle, and both the HCl and the excess metal halide are carried away from the process, typically by a hood or other venting system. The exhaust stream from the venting system is unsuitable for release into the ambient air and must be treated to remove both the metal halides and HCl gas.
For these reasons, it would be desirable to provide a method and apparatus for treating gas streams containing metal halides, particularly gas streams containing both metal halides and HCl gas. Such a method and process should provide for a very high level of removal, preferably above 99%, and should be relatively inexpensive to build and operate.
2. Description of the Prior Art
A rotary spray scrubber of the type of the present invention is described in U.S. Pat. No. 4,242,109 to Edwards. The scrubber includes a conduit for directing the particulate-laden gas stream in a first direction. An axial impeller induces flow of the gas through the conduit. A rotary dispersion device attached on a common shaft with the impeller and located immediately downstream thereof directs a spray of droplets into the flowing air stream. The nature of the rotary dispersion device is not discussed in detail in the patent.
Certain machines incorporating the teachings of the Edwards patent have been manufactured and sold by EMCOTEK Emission Control Technology, Visalia, Calif. 93291. The dispersion device employed in these machines is illustrated in FIG. 1 and comprised a six-inch diameter hub 10 including a pair of spaced-apart disks 12 joined by a perforated sheet metal periphery 14. Perforations 16 comprised about 50% of the peripheral surface area. Water was fed into the hub 10 through an opening 18 in one of the disks and the hub rotated to generate a spray. The hubs were placed in conduits (reference no. 15 in the Edwards patent) having a diameter of 18 inches. Using this device, collection efficiencies of submicron particles above 50% could not be achieved.
Various aqueous scrubbing systems have been proposed for the removal of metal halides from gas streams. U.S. Pat. No. 3,690,041 teaches the use of an aqueous pre-spray to eliminate misting normally encountered in the conventional water scrubbing of gas streams containing both HCl gas and stannic chloride. U.S. Pat. Nos. 3,789,109 and 3,885,929 both teach the high temperature conversion of metal halides to metal oxides prior to conventional scrubbing. See also, Russian Pat. No. 789,139 which concerns the construction of a particular scrubber for the removal of titanium tetrachloride from gas streams. Diffusiophoretic and electrostatic methods for the removal of metal halides from a gas stream are taught by U.S. Pat. Nos. 3,956,532 and 3,967,939. Prior art approaches for the removal of metal halides from glass treatment effluents are described generally in "How to Remove Pollutants and Toxic Materials From Air and Water," Pollution Technology Review, No. 32, Noyes Data Corporation, 1977, pages 208-214.